Pseudowire (PW) is a mechanism that emulates the essential attributes of a telecommunications service (Frame Relay, Asynchronous Transfer Mode, Ethernet) over a Packet Switched Network (PSN). These Layer 2 services can be emulated over an MPLS backbone by encapsulating the layer 2 Protocol Data Units (PDU) and transmitting them with MPLS labels that identifies a PWs. Label Distribution Protocol (LDP) is used as the default protocol for Pseudowire Setup and Maintenance as per Internet Engineering Task Force (IETF) RFC 4447.
The terminology used herein includes PE for “Provider Edge Device” and CE for “Customer Edge Device”. A PSN tunnel is established to provide a data path for the PW between two provider edge devices. PW traffic is not distinguishable to the core network, and the core network is transparent to the CEs. Native data units (bits, cells, or packets) arrive via the respective Attachment Circuit (AC) between each PE and its CE, are encapsulated in a PW PDU, and are carried across the underlying network via the PSN tunnel. The PEs perform the necessary encapsulation and decapsulation of PW PDUs and handle any other functions required by the PW service, such as sequencing or timing.
A Multi-Segment Pseudowire (MS-PW) is a set of two or more contiguous PW segments that behave and function as a single point-to-point μW. A MS-PW enables providers to extend the reach of PWs across multiple transport PSN domains. The terminology used herein includes S-PE for “Switching” PE Device where two segments of a MS-PW are stitched together. PW Labels are switched by S-PE between two PW segments.
RFC4447 describes a concept of PW grouping that represents an arbitrary group of PWs specific to a “Terminating” PE (T-PE) device. T-PE stands for the PE device where a PW terminates. When label mapping messages are exchanged between two PW emulation points, those messages carry a PW grouping TLV that identifies a PW group local to the sender T-PE. The T-PE at the other end (receiver of the label mapping) maintains a database of PWs that map to the PW group at sender T-PE.
The PW grouping allows “wildcard” messaging from the sender T-PE on the entire group when any event common to the group requires notifications to the other end T-PE—such as wildcard label withdrawals or wildcard status notification messages. A single message can be sent with the PW grouping identifier (ID) to notify action on all member PWs in a group. For example, a PW grouping ID can be used as a port index and can be assigned to all PWs that have ACs bound to that port. Use of PW grouping ID enables a PE to send one single wildcard label withdrawal message or PW status notification message specifying the group ID in the event of port failure. Such wildcard messaging provides significant reduction of per PW messaging overhead and makes the PW Operations Administration and Maintenance (OAM) Status notifications very efficient.
The existing method of PW grouping imposes the restriction that a PW can belong to only one group across the T-PEs. Further with MS-PW, it is not possible that a PW can belong to the same group across the T-PEs since the MS-PW traverses across one or more S-PEs and such PW grouping is meaningful across a single PW segment.
A set of PWs originating in a first T-PE can be bound to the same local port. It is not necessary that all member PWs are bound to the same PSN tunnel between the first T-PE and a first S-PE since the PSN requirements of the PWs may vary based on diverse Quality of Service (QoS) or diverse next-hop S-PEs requirements, etc. Two PWs of the group may share the same local port but may be routed to different S-PEs. This requires a PW to assign at least two groups at the sender T-PE:                1. An “access” group based on port id or other attributes on the attachment circuit (AC) for triggering a wildcard PW status message on various fault/status transitions associated with the AC.        2. A “network” group based on the PSN tunnel used to reach the first S-PE for triggering wildcard PW status message on various PSN fault/status transitions associated with the PSN tunnel.        
The grouping association may also change at each S-PE along the MS-PW. For example, the first S-PE may receive a PW set-up request for both PWs, which have the same grouping ID “G”, but may route each of the PWs to different S-PEs or the same S-PE but over different PSN tunnels based on diverse QoS/Policy requirements of each of the PWs. However in both cases, if the first S-PE detects a PSN tunnel fault towards another S-PE, it cannot use wildcard messaging to notify the fault to all PEs of the member PWs that are affected. Further, when an S-PE receives a wildcard message with group G, it cannot transparently forward the wildcard message to next-hop S-PE(s) since grouping ID is meaningful only for a single PW hop.
For efficiency and scalability of PW maintenance, it is required that wildcard messaging be possible from any T-PE or S-PE that can be seamlessly notified across all T-PE or S-PE devices through which the PWs that belong to the group traverse. Hereinafter this document at times may use PE to specify a T-PE or an S-PE. LDP is a Transmission Control Protocol (TCP) based protocol which is prone to signaling delays due to congestion control in TCP. In Dynamic MS-PW, a PW status message traverses through control planes of each of the S-PEs of the MS-PW since that is a requirement of PW OAM Message Mapping, according to IETF RFC6310. Efficiency and performance of PW status signaling is a very important factor for PW OAM Message Mapping, PW Redundancy, PW applications such as Virtual Private Local Area Network Service (VPLS) Media Access Control (MAC) Address Flush for example according to IETF RFC4762. When PW scaling requirements are high (e.g. 128,000 PWs) the resulting high volume of PW status signaling may impact the operational efficiency of LDP and SLA (Service Level Agreement) of the PW service,
Therefore, an efficient way of establishing and managing the PW groups and conveying information in connection with the groups to devices in the PSN is desired.